To address widespread concerns about cancer risks from residential magnetic field exposures, investigators from NCI and from the Children's Cancer Group have conducted a case-control study of childhood acute lymphoblastic leukemia. Three recent analyses focused on methodologic issues of exposure metrics. In particular, we evaluated metrics for magnetic field levels (including threshold values, peak values, rate of change, and other metrics); distance and load of power lines in relation to residences; and possible effects of selection bias and potential confounders of residential magnetic field measurements (the latter including sociodemographic factors). Most metrics were positively correlated with a time-weighted average metric; and the time-weighted average metric was not associated with leukemia risk. In addition, magnetic fields from appliances were measured to determine if specific types of appliances generated higher magnetic field exposures. Findings were negative. The etiology of brain tumors is poorly understood, and recorded incidence rates have increased dramatically over the past several decades. Whether this increase is, in part, real or is entirely an artefact of improved diagnosis is a controversial issue. Nonetheless, concern has arisen that one or more increasingly common environmental exposures might cause brain cancer. Examples include industrial chemicals, pesticides, food additives, and electromagnetic fields. In response to such concerns, and to advance understanding of environmental, behavioral and genetic causes of brain tumors, we are collaborating with investigators at three U.S. hospitals in conducting a case-control study of malignant and benign brain tumors. Factors under consideration include workplace exposures to chemical agents and electromagnetic fields, use of cellular telephones, dietary factors, family history of tumors, genetic determinants of susceptibility, home appliance use, reproductive history and hormonal exposures, viruses, medical and dental exposure to ionizing radiation, and other aspects of medical history. Key features of the study include its large size, the emphasis on rapid ascertainment of incident cases and interview of study subjects rather than surrogate respondents, the use of detailed, job-specific questions developed by industrial hygienists to ascertain occupational exposures, and the storage of blood samples for future evaluation of inherited susceptibility, biomarkers of exposure, and gene-environment and gene-gene interactions. Data analysis is underway.In response to concerns about cancer risk from non-ionizing radiation of microwave and radio frequencies, an earlier study of cancer mortality among Korean War Naval Service veterans exposed to microwave radiation has been reactivated. Two cohorts of 20,000 men each, who served as shipboard radar operators and maintenance workers, respectively, during their Navy service were followed for cancer mortality through 1993. Analysis results on the relationship between cancer risk and non-ionizing radiation dose are expected in late 2000.